In operating machines, each part dimension (a dimension is a numerical value that defines the size, shape or location of a feature) and the distance between spatially-opposed parts (i.e., a gap) must be maintained within design tolerances for efficient and reliable machine operation. For example, an aircraft engine includes between about 10,000 and 40,000 parts, each described by one or more two-dimensional drawings illustrating the part geometry, nominal part dimensions, part tolerances and other required part characteristics (e.g., material from which the part is to be constructed and non-observable properties such as tensile strength or hardness). The tolerance values specify an acceptable range of variation from the nominal part dimension.
Each part is manufactured in accordance with the information set forth in the drawings (i.e., each drawing is a two-dimensional representation of the part), including conformance to the part nominal dimensions and the tolerances associated with each dimension. A part having an actual dimension outside the specified tolerance range may not be suitable for use in the machine as it may not properly mate or interface with another part or may physically interfere with the operation of another part. A single part can be governed by a plurality of tolerances, each tolerance related to a different geometric feature of the part. For example, the drawings for an aircraft engine gas turbine blade may include as many as four hundred dimension tolerances.
As the individual parts are assembled to form the machine, gaps (defined generally as a linear distance between two planes or surfaces in the machine) between parts must be maintained within design limits. A nominal gap distance represents the desired gap opening. A gap tolerance indicates a range of acceptable variations from the nominal gap distance. For example, in a gas turbine jet engine combustion gases impinge upon a plurality of blades carried by a spinning rotor enclosed within a stationary stator. Maintaining a specified gap (as defined by the nominal gap distance and a tolerance range associated with the gap distance) between a tip of each rotor blade and the stator is necessary for proper and efficient operation of the engine. Typically, the gap tolerance is measured in ten-thousandths of an inch. A gap outside the tolerance range by one ten-thousandth of an inch can significantly affect several important operational parameters, such as engine fuel efficiency.
Dimension tolerance stack-up analysis is a process of using given machine part dimensions and part tolerances to predict the dimension and tolerance of an assembly dimension between two mating or adjoining parts, e.g., to predict the nominal dimension and tolerance of a machine gap. The gap between the blade tip and the stator is an example of a gap subject to stack-up analysis. Critical gaps are subject to stack-up analysis during the machine design phase to ensure proper machine operation. In a typical steam turbine there are about 130-180 critical gap stack-ups that can affect turbine performance. In a typical aircraft engine there are more than 2000 such stack-ups.
The stack-up analysis is important for improving quality and reducing production costs o the machine. Design engineers favor a tight tolerance on each machine dimension, requiring close dimensional control during the manufacturing process. But the dimensional control adds costs to the manufacturing process. Thus a quantitative analysis of the machine dimensions and the tolerance associated with each dimension provides important insight into design and manufacturing processes, allowing informed trade-offs between their competing interests.
To perform the gap stack-up analysis and thereby determine the nominal gap dimension and the gap tolerance for a given machine gap, it is necessary to first identify the interfacing or mating parts and the dimensions of those parts that create the gap, i.e., a gap vector loop. These part dimensions form a loop beginning at one gap surface, traversing through serial part interfaces until reaching the opposing gap surface. The part dimensions and part dimension tolerances associated with each such loop part are then combined to yield the gap nominal dimension and the gap tolerance.
According to the prior art, the gap stack-up analysis process is performed manually. A gap of interest is identified and the drawing(s) for each part that affects the gap dimension is retrieved. Each part drawing is examined to determine the nominal part dimensions that affect the gap and the tolerance associated with each such dimension. The nominal part dimensions and part dimension tolerances are recorded. Each dimension is further assigned a multiplication factor (typically +1 or −1) based on whether the dimension value increases or decrease the gap dimension.
The manual gap stack-up process is complicated by the necessary reliance on two-dimensional drawings to determine contributions to a three-dimensional gap. The engineer performing the stack-up process must therefore have considerable familiarity with the machine and the spatial relationship of its constituent parts to perform an accurate stack-up measurement. The manual stack-up process is extremely time-consuming, usually requiring between about 10 and 30 hours of analysis for one machine gap. Mistakes are easily made, as the number-intensive nature of the process is prone to errors, such as the transposition of two adjacent digits in a tolerance value. Also, accurate extraction of the vector loop is a difficult process to perform manually. This process is further complicated if multiple vector loops exist. In these circumstances, the engineer may be unaware of the existence of such multiple vector loops and extraction of all vector loops is not performed, which results in inaccurate stack-up results. Once completed, it is desired to validate the vector loop (i.e., the parts and their surfaces that form the vector loop) and the stack-up results. However, there are no known validation techniques, short of having the stack-up analysis repeated by another engineer.
Today, most machines and their constituent parts are designed using computer-aided design (CAD) software that creates three dimensional images or models of each part, including nominal part dimensions, but typically excluding tolerance values for the nominal dimensions. Upon completion of the design, a prototype machine is assembled from prototype parts based on the software-generated design data. Only later, after the two-dimensional drawings are created for each part and the dimension tolerances determined, can an engineer perform a stack-up analysis. However; given the protracted process for creating the two dimensional drawings and conducting a stack-up analysis, the analysis may not be completed until after prototype parts have been assembled into a prototype machine. The lack of a tolerance stack-up analysis during part fabrication and machine assembly may result in a prototype machine that does not perform as expected due to out-of-tolerance part gaps. Upon discovery of an out-of-tolerance gap, the affected part tolerances must be modified, the part refabricated in accordance with the corrected tolerance values and assembled into the machine. Correction of these problems during the design and fabrication cycle adds unwanted costs to the product, and correction costs increase as the design and fabrication cycle nears an end.